Free Drainage Fixture Unit (DFU) Calculator
Calculate total drainage fixture units (DFUs), size horizontal drain branches, building sewer pipes, and vertical stacks per IPC and UPC codes.
🚿 Plumbing Fixtures Inventory
Toilets: 9 DFU • Lavs: 4 DFU • Baths: 6 DFU • Kitchen: 2 DFU • Laundry: 2 DFU • DW: 2 DFU
• Water Closet Branch: Must be at least 3" minimum diameter (never downsize a toilet drain!).
• Washing Machine Standpipe: Requires 2" minimum trap and drain pipe (cannot use 1.5").
The Self-Scouring Velocity Rule: Horizontal drain lines are engineered for a slope of 1/4" per foot to maintain a water flow velocity of 2.0 to 2.5 feet per second. At this velocity, water carries suspended solid waste without stranding solids behind.
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What is a Drainage Fixture Unit (DFU)?
A Drainage Fixture Unit (DFU) is an engineering metric representing the probable discharge of water into the drainage system by various plumbing fixtures. Formulated by Dr. Roy B. Hunter at the National Bureau of Standards, fixture units account for the statistical probability that fixtures will be operated simultaneously.
Standard Fixture DFU Values (IPC vs. UPC)
- Water Closet (Flush Tank): 3 DFU (IPC) / 4 DFU (UPC) • Minimum 3" pipe.
- Bathroom Lavatory Sink: 1 DFU • Minimum 1-1/4" or 1-1/2" pipe.
- Shower Stall: 2 DFU • Minimum 2" pipe trap.
- Bathtub (with or without shower): 2 DFU • Minimum 1-1/2" pipe.
- Kitchen Sink: 2 DFU • Minimum 1-1/2" or 2" pipe.
- Laundry Washing Machine: 2 DFU (IPC) / 3 DFU (UPC) • Minimum 2" standpipe.
Frequently Asked Questions
Why can’t I slope my drain pipe steeper, like 1 inch per foot?
If a drain pipe is sloped too steeply (> 1/2" per foot), liquids run down the pipe too fast, leaving solid wastes behind on the dry pipe invert where they clump, dry out, and cause recurring clogs.
Can a 3" pipe handle a 4-bathroom house?
Yes! A 3" horizontal building drain sloped at 1/4" per foot is rated to carry up to 42 DFU under IPC, which comfortably accommodates 4 full bathrooms (approx 28-35 DFU total).
When is a 4" main building drain required?
A 4" main drain is required whenever total fixture loading exceeds 42 DFU (at 1/4" slope) or when local municipal codes mandate 4" from the foundation wall out to the street sewer.